As a semiconductor material applicable to a transistor, an oxide has been attracting attention. For example, Patent Document 1 discloses a field-effect transistor including an amorphous oxide of any of an In—Zn—Ga—O-based oxide, an In—Zn—Ga—Mg—O-based oxide, an In—Zn—O-based oxide, an In—Sn—O-based oxide, an In—O-based oxide, an In—Ga—O-based oxide, and a Sn—In—Zn—O-based oxide.
Non-Patent Document 1 discusses a structure in which an active layer of a transistor includes two layers of metal oxides of an In—Zn—O-based oxide and an In—Ga—Zn—O-based oxide.